The present invention relates generally to processes for producing mirrors, and more particularly to an improved process for producing flat or parabolic mirrors for visible wavelength applications using an innovative polymeric casting process.
Prior art processes for producing flat and/or parabolic mirrors (especially large aperture mirrors) for visible wavelength applications have long consisted essentially of rough machining a material (glass, metal or ceramic) to a desired surface shape, grinding to a refined shape, and polishing the surface to a final desired accuracy. These processes are typically expensive and labor intensive, and more recently developed processes have resulted in only incremental improvements in production time and cost. Recently, a process known as spin casting was applied to the production of high quality parabolic mirror systems at greatly reduced cost.
Spinning a contained liquid in a static gravity environment produces a parabolic surface. The spinning process was applied in the 20th century for producing mirrors first by Wood (Astrophysical Journal, 29, 164 (1908)) and later by Borra (“Liquid Mirrors,” Scientific American (Feb. 1994), and “The Case for Liquid Mirrors in Orbiting Telescopes,” Astrophyical Journal, 392(1) (Jun. 1992)); Gibson (“Liquid Mirror Telescopes: History,” Journal of the Royal Astronomical Society of Canada, 85(4) (Aug. 1991); and Borra et al, “Liquid Mirrors: Optical Shop Tests and Contributions to the Technology,” Astrophysical Journal, 393(2) (July 1992)). The parabolic mirrors were produced using liquid mercury as a reflective liquid and demonstrated excellent surface characteristics, but required the mirror system to remain spinning and pointing only directly upwardly and depended on the use of (highly toxic) mercury. More recently, Alvarez et al (“Large Off-Axis Epoxy Paraboloids for Millimetric Telescopes and Optical Light Collectors,” Review of Scientific Instruments, 64(1) (Jan. 1993)) used a polymer resin to spin-cast a 1.75 meter diameter infrared telescope mirror. The mirror had a surface accuracy of 25 microns (μm) RMS and was adequate for the desired application. The Steward Observatory Mirror Laboratory of the University of Arizona successfully produced large glass mirrors using the spin-casting method in which a spinning furnace is used to melt the glass and promote a parabolic shape. The surface is, however, inadequate for visual wavelength applications and requires labor and time intensive grinding and polishing.
The invention solves or substantially reduces in critical importance problems with prior art processes for producing mirrors as just described by spin casting or settle casting a liquid polymer resin onto a rigid or semi-rigid substrate. The resin becomes the mirror surface and the substrate provides the rigidity to support visual wavelength mirrors. In the settling process, the liquid resin is applied to a substantially flat substrate in a uniform acceleration (gravity) environment, and results in a flat mirror surface upon resin solidification. In spin casting, the liquid resin is applied to a substrate having substantially parabolic shape while the substrate is spinning about a vertically aligned axis, and results in a parabolic mirror surface upon resin solidification. Spinning the liquid resin produces a parabolic surface regardless of the underlying substrate surface geometry, and the surface shape of the polymer resin is retained after cure. A reflective coating may then be applied to the solid resin surface to define the desired parabolic reflector.
According to a principal feature of the invention, multiple layers of resin are applied to the substrate because resin shrinkage during solidification and cure causes the finished mirror surface to mimic topographic flaws in the surface of the substrate. If shrinkage is small, the mimicked flaws are smaller than the flaws on the substrate. Additional layers of resin reduce the topography of the flaws until the desired surface accuracy is achieved. The multi-layering aspect of the invention obviates any need for expensive machining to achieve optical tolerances and therefore greatly reduces the cost of producing flat or parabolic mirrors.
It is a principal object of the invention to provide an improved process for producing flat or parabolic mirrors.
It is a further object of the invention to provide a process for producing mirrors by casting a liquid polymer resin.
It is another object of the invention to provide an improved process for producing mirrors from liquid polymer resins using a spin casting process.
It is yet another object of the invention to provide a process for producing mirrors in a wide size range having a high degree of surface accuracy.
It is yet another object of the invention to provide a process for producing optical quality mirrors without the need for substantial final polishing.
It is yet another object of the invention to provide a low cost process for producing mirrors of high optical quality.
It is another object of the invention to provide a process for producing lightweight mirrors.
These and other objects of the invention will become apparent as a detailed description of representative embodiments proceeds.